The idea that we can just keep growing forever on a finite planet is totally imbecilic.

— Paul Ehrlich

Most people are familiar with the concept of limits to growth, thanks to the dedicated writing and activism of visionaries such as Donella Meadows, Paul and Anne Ehrlich, Eugene Odum, and Lester Brown to highlight but a few. The concept, which states simply that growth in human societies depends on the Earth's finite resources, is more or less just an application to human societies of the first law of thermodynamics, which states that in a closed system matter/energy can neither be created nor destroyed.

A common way of expressing limits is carrying capacity, an elegant but contested concept that has origins in the ranching science of the late 1800s. Ecologist Eugene Odum is largely responsible for explicating carrying capacity as it is most commonly used by scientists and laypersons. Carrying capacity is the maximum number of organisms, whether cows or salmon or people, that any given landscape or seascape can support. In the presence of surplus resources, populations grow, usually exponentially, but this growth eventually slows in response to environmental pressures (increasing scarcity). Ultimately, populations reach a "steady state", in which they oscillate around the carrying capacity as a sort of asymptote (see figure).

Carrying capacity as a hypothetical biological pattern is contested among scientists; another respected ecologist named G.E. Hutchinson called the concept "theoretically elegant but empirically vacuous." Its failing, as with many scientific models, lies in is its (over)simplicity. Populations of animals rarely, if ever, follow such a simple mathematical pattern because ecosystems are extraordinarily complex. Predator-prey relationships, ecological invasions, prey switching, and niche construction all stymie ecologists' attempt to find clear-cut, empirical evidence of carrying capacity as mapped out by Odum.

Nevertheless, use of the concept remains widespread in conservation and fish and game management and largely because of its elegance; the notion of Maximum Sustainable Yield (MSY) as applied in commercial fisheries is more or less just an application of carrying capacity to human activities, and its use continues to be widespread despite harsh critiques by scientists such as Peter Larkin being sounded as much as 40 years ago.

The limits to growth hypothesis has also been extensively contested by economists. Human innovation, some say, will always allow us to avoid any limits to growth through the identification of new substitutions for dwindling resources. This argument is a bit fallacious, guilty of moving the goalpost", and as such it doesn't really even deny the existence of physical limits but simply rejects the notion that fully depleting any particular resource is a problem. In other words, from this perspective, there is nothing wrong with running out of oil, or redwood trees, or salmon, because humans will always find something else to exploit in their place.

I think most people are on board with a definition of sustainability that does not entail systematically depleting each of the world's renewable and non-renewable resources. What would the Pacific Northwest be without salmon or redwoods? Or Maine without lobster? These are cultural, not economic arguments, but only the most obdurate stalwarts I think continue to deny these as legitimate examples of the importance of the concept of limits. In part, this is also because the conversation about limits and sustainability more generally has moved away from the challenge of locating specific biophysical limits, and to the task of understanding the implications of their existence for the management of valued resources.

For example, ecological economist Herman Daly, the modern father of the concept of the "steady state economy", offers three rules for sustainability that derive logically from the premise of limits:

Renewable resources (fish, soil, and groundwater) must not be used faster than the rate at which they regenerate. (For example, fish stocks may not be fished at a rate greater than the fish can reproduce);

Nonrenewable resources (minerals and fossil fuels) must not be used faster than the rate at which renewable substitutes can be put into place. (For example, reinvesting profits from fossil fuel enterprises into solar or bio-fuel);

Pollution and wastes must not be emitted faster than natural systems can absorb, recycle, or render them inert. (If sewage is released into a lake or river, it must not be done faster than aquatic life can absorb the nutrients).

Limits to Limits

Who wants to be told not to dream big?

Each of Daly's rules derives in some way from the notion of limits and the value that sustainability requires conservation of specific resources, rather than just using them up and moving on to the next.

Yet, the trouble with limits is that they provide a terrible metaphor for sustainability. They set sidebars for our actions by telling us what we can't do--harvest more than x fish or y trees. They don't tell us what we can do or should do or even might do. Many would also argue that in a society characterized by immense ecological overshoot, limits demand extreme restraint and austerity; while important, these do not provide an exciting vision for the future that all people can get behind. This very point, arguably, is the most poignant criticism of the contemporary environmental movement: who wants to be told to not dream big?

Of course, any proponent of steady state economics will note that the alternative, continued growth, has a poor pedigree with respect to outcomes that people care about: happiness, health, equity. I don't disagree with Daly's thesis about a steady state economy being essential to a sustainable, equitable, and verdant society. However, the steady state economy is the pattern of sustainability, not the behavior that creates it. Similarly, limits, while an inescapable truth, are not the metaphor by which we can redesign our behavior in a fashion that gets us there.